The present invention relates to prestressed universal rolling mills adapted for rolling H-section and other shape steels of various sizes.
Rolling mills of this type must meet such requirements as simple and lightweight construction, low cost, reduced roll changing time and high mill rigidity, and a variety of rolling mills of this type have been proposed. For instance, with a known rolling mill of this type in which an upper cross beam carrying a top horizontal roll and a lower cross beam carrying a bottom horizontal roll are respectively arranged above and below an intermediate frame securely held on a sole plate and equipped with vertical rolls, and the upper and lower cross beams are held together by means of four tie rods, the mill stiffness relating to the horizontal rolls is largely dependent on the elongation of the tie rods so that if the rod diameter is 220 mm, the rod length is 2,600 mm, the roll diameter is 1,200 mm, the size of an H-section to be rolled is 500 mm .times. 200 mm .times. 10 mm .times. 16 mm and the maximum rolling reaction force is 1,200 tons, then the resulting elongation is 0.98 mm or 0.08 mm per 100 tons. In the case of an conventional rolling mill, the corresponding elongation is about 0.02 mm per 100 tons, and this is far smaller than that of the rods in the above-mentioned universal rolling mill. In other words, while the mill rigidity of the above-mentioned universal rolling mills is low and this low mill rigidity may be improved by increasing the diameter of the tie rods, it is difficult to do so in consideration of the structure of this type of rolling mills. In addition, since there are instances where the maximum rolling force or load during a rolling operation amounts to 1,200 tons and the resulting elongation of the tie rods also amounts to 0.98 mm as mentioned previously, a clearance is produced between the intermediate frame and each of the upper and lower cross beams with the result that the position of the rolls carried on these beams and frame tend to be easily shifted, and hence the dimensional accuracy of products is adversely affected. Still further, while it is generally necessary to provide a breaker to prevent the occurrence of any serious accident due to a jamming or the like that will be caused by erroneous settings of the roll clearance or the like, the provision of such a breaker block also tends to result in a deteriorated mill stiffness.